1. Field of the Invention
The present invention relates to a lithographic projection apparatus. More specifically, the present invention relates to a substrate support for a lithographic projection apparatus.
2. Description of Related Art
In order to achieve good image definition and layer overlay the irradiated surface of a substrate should be kept as flat and as stationary as possible during exposure of the substrate. Known lithographic apparatus addresses these demands using the substrate holder hereabove specified, on which a substrate can be placed so that its backside is in contact with the protrusions, all of which lie in a well-defined plane. For example, by connecting aperture(s) in the substrate holder to a vacuum generating device, the backside of the substrate can be clamped securely against the protrusions. The use of protrusions in this manner ensures that only a fraction of the area of the backside is actually pressed against a solid surface; in this way, the distorting effect of any particulate contamination on the backside of the wafer is minimized, since such contamination will most probably be situated in the empty spaces between protrusions rather than being pressed against the top surface of a protrusion.
Also, in this way, substantially every protrusion contacts the substrate, thus defining one single plane of support that is perfectly level and orientable into required directions. This also poses a problem since there remains a relatively large contact surface of the protrusions physically in contact with the substrate. This may introduce a binding force, in the art known as “sticking”, between the top faces of the protrusions of the substrate holder and the backside of substrate. In practice, this means that once a substrate is clamped to the substrate holder and brought into position for a photolithographic irradiation process, releasing the substrate from the substrate holder can take a substantial amount of time causing costly delay in the availability of the machine for a next photolithographic routine. It even may cause jamming of the ejection mechanisms present to release the substrate from the substrate holder.